docs: def serialize_date(attr, **kwargs):
          """Serialize Date object into ISO-8601 formatted string.
  
          :param Date attr: Object to be serialized.
          :rtype: str
          """
          if isinstance(attr, str):
              attr = isodate.parse_date(attr)
          t = "{:04}-{:02}-{:02}".format(attr.year, attr.month, attr.day)
          return t